disallow implicit return if result is used, don't use void context

compiler/semexprs.nim

@@ -2061,7 +2061,6 @@ proc semAsgn(c: PContext, n: PNode; mode=asgnNormal): PNode =
     let lhs = n[0]
     let rhs = semExprWithType(c, n[1], {efTypeAllowed}, le)
     if lhs.kind == nkSym and lhs.sym.kind == skResult:
-      n.typ() = c.enforceVoidContext
       if c.p.owner.kind != skMacro and resultTypeIsInferrable(lhs.sym.typ):
         var rhsTyp = rhs.typ
         if rhsTyp.kind in tyUserTypeClasses and rhsTyp.isResolvedUserTypeClass:
@@ -2092,10 +2091,12 @@ proc semReturn(c: PContext, n: PNode): PNode =
   if c.p.owner.kind in {skConverter, skMethod, skProc, skFunc, skMacro} or
       (not c.p.owner.typ.isNil and isClosureIterator(c.p.owner.typ)):
     if n[0].kind != nkEmpty:
+      var removeResultSymUsed = false
       if n[0].kind == nkAsgn and n[0][0].kind == nkSym and c.p.resultSym == n[0][0].sym:
         discard "return is already transformed"
       elif c.p.resultSym != nil:
         # transform ``return expr`` to ``result = expr; return``
+        removeResultSymUsed = sfUsed notin c.p.resultSym.flags
         var a = newNodeI(nkAsgn, n[0].info)
         a.add newSymNode(c.p.resultSym)
         a.add n[0]
@@ -2104,6 +2105,8 @@ proc semReturn(c: PContext, n: PNode): PNode =
         localError(c.config, n.info, errNoReturnTypeDeclared)
         return
       result[0] = semAsgn(c, n[0])
+      if removeResultSymUsed:
+        c.p.resultSym.flags.excl sfUsed
       # optimize away ``result = result``:
       if result[0][1].kind == nkSym and result[0][1].sym == c.p.resultSym:
         result[0] = c.graph.emptyNode
@@ -2116,7 +2119,10 @@ proc semProcBody(c: PContext, n: PNode; expectedType: PType = nil): PNode =
       return n
   openScope(c)
   result = semExpr(c, n, expectedType = expectedType)
-  if c.p.resultSym != nil and not isEmptyType(result.typ):
+  if c.p.resultSym != nil and
+      not isEmptyType(result.typ) and
+      sfUsed notin c.p.resultSym.flags:
+    incl(c.p.resultSym.flags, sfUsed)
     if result.kind == nkNilLit:
       # or ImplicitlyDiscardable(result):
       # new semantic: 'result = x' triggers the void context

compiler/semstmts.nim

@@ -1897,7 +1897,6 @@ proc addResult(c: PContext, n: PNode, t: PType, owner: TSymKind) =
     var s = newSym(skResult, getIdent(c.cache, "result"), c.idgen,
                    getCurrOwner(c), n.info)
     s.typ = t
-    incl(s.flags, sfUsed)
 
   if owner == skMacro or t != nil:
     if n.len > resultPos and n[resultPos] != nil:

lib/packages/docutils/rst.nim

@@ -2842,7 +2842,7 @@ proc readTableRow(p: var RstParser): ColSeq =
 proc getColContents(p: var RstParser, colLim: ColumnLimits): string =
   for i in colLim.first ..< colLim.last:
     result.add(p.tok[i].symbol)
-  result.strip
+  result = result.strip
 
 proc isValidDelimiterRow(p: var RstParser, colNum: int): bool =
   let row = readTableRow(p)

tests/discard/tvoidcontext2.nim

@@ -0,0 +1,13 @@
+discard """
+  errormsg: '''expression '"invalid"' is of type 'string' and has to be used (or discarded)'''
+  line: 11
+"""
+
+proc foo(x: var string) =
+  x = "hello"
+
+proc bar(): string =
+  foo(result)
+  "invalid"
+
+echo bar()

tests/discard/twrongvoidcontext.nim

@@ -0,0 +1,12 @@
+discard """
+  output: '''
+true
+'''
+"""
+
+# issue #16855
+
+proc f(): int =
+  echo (result = 1; result > 0)
+
+doAssert f() == 1